Enable systemd hardening options for named

Ludovic Gasc gmludo at gmail.com
Tue Jan 16 11:47:45 UTC 2018


Hi,

I have forgotten to say that I have also removed "-u bind" option in
/etc/default/bind9, because it isn't necessary anymore: The named daemon is
started as bind user directly with this configuration.

I might found 3 new interesting options:
https://gist.github.com/ageis/f5595e59b1cddb1513d1b425a323db04
SystemCallArchitectures=native
MemoryDenyWriteExecute=true
RestrictRealtime=true


--
Ludovic Gasc (GMLudo)

2018-01-16 12:21 GMT+01:00 Ludovic Gasc <gmludo at gmail.com>:

> Hi,
>
> I have merged config files from Tony, Robert, and me.
> I have tried to be the most generic, the result below.
>
> It seems to work here without regression, except a warning:
> managed-keys-zone: Unable to fetch DNSKEY set '.': operation canceled
>
> But only at the first boot, I don't see the message anymore when I restart
> the daemon.
> Any clue ?
>
> Thanks for your feedbacks.
>
> [Unit]
> After=network-online.target
>
> [Service]
> Type=simple
> TimeoutSec=25
> Restart=always
> RestartSec=1
> User=bind
> Group=bind
> CapabilityBoundingSet=CAP_NET_BIND_SERVICE
> AmbientCapabilities=CAP_NET_BIND_SERVICE
> SystemCallFilter=~@mount @debug acct modify_ldt add_key adjtimex
> clock_adjtime delete_module fanotify_init finit_module get_mempolicy
> init_module io_destroy io_getevents iopl ioperm io_setup io_submit
> io_cancel kcmp kexec_load keyctl lookup_dcookie migrate_pages move_pages
> open_by_handle_at perf_event_open process_vm_readv process_vm_writev ptrace
> remap_file_pages request_key set_mempolicy swapoff swapon uselib vmsplice
>
> NoNewPrivileges=true
> PrivateDevices=true
> PrivateTmp=true
> ProtectHome=true
> ProtectSystem=strict
> ProtectKernelModules=true
> ProtectKernelTunables=true
> ProtectControlGroups=true
> InaccessiblePaths=/home
> InaccessiblePaths=/opt
> InaccessiblePaths=/root
> ReadWritePaths=/run/named
> ReadWritePaths=/var/cache/bind
> ReadWritePaths=/var/lib/bind
>
>
> --
> Ludovic Gasc (GMLudo)
>
> 2018-01-15 21:14 GMT+01:00 Robert Edmonds <edmonds at mycre.ws>:
>
>> Tony Finch wrote:
>> > Ludovic Gasc <gmludo at gmail.com> wrote:
>> > >
>> > > 1. The list of minimal capabilities needed for bind to run correctly:
>> > > http://man7.org/linux/man-pages/man7/capabilities.7.html
>> >
>> > named already drops capabilities - have a look at the code around here:
>> > https://source.isc.org/cgi-bin/gitweb.cgi?p=bind9.git;a=blob
>> ;f=bin/named/unix/os.c;hb=v9_11_2#l234
>> >
>> > Note that it's a bit clever - the privileges are dropped in two stages,
>> > right at the start, and after the server has been configured.
>>
>> I checked just now to see what that code actually ends up doing, and on
>> my system I ended up with:
>>
>>     $ grep -h ^Cap /proc/$(pidof named)/**/status | sort | uniq -c
>>           6 CapAmb:     0000000000000000
>>           6 CapBnd:     0000003fffffffff
>>           6 CapEff:     0000000001000400
>>           6 CapInh:     0000000000000000
>>           6 CapPrm:     0000000001000400
>>     $
>>
>> That decodes to:
>>
>>  - The effective and permitted capabilities sets were reduced to
>>    CAP_NET_BIND_SERVICE and CAP_SYS_RESOURCE.
>>
>>  - The ambient and inheritable capabilities sets were cleared.
>>
>>  - The capability bounding set was left completely open-ended.
>>
>> It's not clear why CAP_SYS_RESOURCE needs to be retained past startup:
>>
>>         /*
>>          * XXX  We might want to add CAP_SYS_RESOURCE, though it's not
>>          *      clear it would work right given the way linuxthreads work.
>>          * XXXDCL But since we need to be able to set the maximum number
>>          * of files, the stack size, data size, and core dump size to
>>          * support named.conf options, this is now being added to test.
>>          */
>>         SET_CAP(CAP_SYS_RESOURCE);
>>
>> See commits 5e4b7294d88ab58371d8c98e05ea80086dcb67cd,
>> 108490a7f8529aff50a0ac7897580b59a73d9845. "[T]o test"?
>>
>> CAP_SYS_RESOURCE is documented as permitting:
>>
>>    CAP_SYS_RESOURCE
>>           * Use reserved space on ext2 filesystems;
>>           * make ioctl(2) calls controlling ext3 journaling;
>>           * override disk quota limits;
>>           * increase resource limits (see setrlimit(2));
>>           * override RLIMIT_NPROC resource limit;
>>           * override maximum number of consoles on console allocation;
>>           * override maximum number of keymaps;
>>           * allow more than 64hz interrupts from the real-time clock;
>>           * raise msg_qbytes limit for a System V message queue above  the
>>             limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
>>           * allow  the  RLIMIT_NOFILE resource limit on the number of "in-
>>             flight" file descriptors to  be  bypassed  when  passing  file
>>             descriptors  to  another process via a UNIX domain socket (see
>>             unix(7));
>>           * override the /proc/sys/fs/pipe-size-max limit when setting the
>>             capacity of a pipe using the F_SETPIPE_SZ fcntl(2) command.
>>           * use  F_SETPIPE_SZ to increase the capacity of a pipe above the
>>             limit specified by /proc/sys/fs/pipe-max-size;
>>           * override /proc/sys/fs/mqueue/queues_max  limit  when
>> creating
>>             POSIX message queues (see mq_overview(7));
>>           * employ the prctl(2) PR_SET_MM operation;
>>           * set  /proc/[pid]/oom_score_adj to a value lower than the value
>>             last set by a process with CAP_SYS_RESOURCE.
>>
>> I would guess that retaining CAP_NET_BIND_SERVICE and CAP_SYS_RESOURCE
>> during the process runtime permits open-ended reloading of the config at
>> runtime (e.g., binding to a new IP address on port 53 without needing to
>> restart the daemon). So even though BIND drops some capabilities, it's
>> still running with elevated privileges compared to a traditional
>> non-root user.
>>
>> systemd permits a nice pattern for network daemons that want to run as
>> an unprivileged user, but bind to a privileged port (and without using
>> socket activation), without starting the process as root. Basically, you
>> put something like this in the unit file:
>>
>>     [Service]
>>     User=…
>>     Group=…
>>     CapabilityBoundingSet=CAP_NET_BIND_SERVICE CAP_SYS_CHROOT CAP_SETPCAP
>>     AmbientCapabilities=CAP_NET_BIND_SERVICE CAP_SYS_CHROOT CAP_SETPCAP
>>>>
>> Any needed filesystem directories and permissions need to be set up
>> correctly before hand. The service is started by the init system as the
>> unprivileged User/Group specified in the unit file, so there's no need
>> to change UID/GID. CAP_NET_BIND_SERVICE is then used to bind to a
>> privileged port, CAP_SYS_CHROOT is used to perform the chroot, and
>> CAP_SETPCAP is used to drop all remaining capabilities from the
>> capability sets and the capability bounding set, so you end up with a
>> completely unprivileged process at runtime. (Alternatively you could
>> keep CAP_NET_BIND_SERVICE and drop CAP_SYS_CHROOT and CAP_SETPCAP, if
>> you wanted to retain the capability to perform privileged binds at
>> runtime. Or you could eliminate CAP_SYS_CHROOT and use other systemd
>> functionality to make parts of the filesystem inaccessible, etc.) This
>> pattern might be a bit hard to retrofit into BIND at this point, though,
>> other than by adding more knobs.
>>
>> --
>> Robert Edmonds
>> _______________________________________________
>> Please visit https://lists.isc.org/mailman/listinfo/bind-users to
>> unsubscribe from this list
>>
>> bind-users mailing list
>> bind-users at lists.isc.org
>> https://lists.isc.org/mailman/listinfo/bind-users
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.isc.org/pipermail/bind-users/attachments/20180116/89d7a156/attachment-0001.html>


More information about the bind-users mailing list